Let's say the billing amount was 200 dollars, and they gave you only 160 dollars. Then you could say: The amount we received is 40 USD less than the billing amount. If the bill (at a restaurant) is 100 dollars, and you only have 60 dollars, then you would say: The amount we have is 40 USD less than the bill. It is better to use "have" and "received" then "got," since "got" can refer to both, causing confusion.

Hello Clive: I think the general custom, regarding " USD40" is to write it this way; 40 USD, meaning "40 U. S. Dollars". Your sentence; "The amount we got is less USD40 than the billing."; could also be expressed this way; "We got 40 USD less than the billing." Examples of the use of "less than" in sentences: "I am less than enthusiastic about a 6 hour train ride." "My income is less than that of my brother." "It is a hell of a lot less than enough to meet our immediate need. "The public recognition I received is a lot less than I expected. "The amount of mail I received is much less than I expected."

I understand your sentence but it is not correct. Some examples: My exam result was less than I thought I should get. My sister has less money than me. We had $40 less than the bill.